As is known in the art, single-ended data transmission uses only one signal line, with its voltage potential referred to ground. While the signal line provides the forward path for signal currents, ground provides the return current path. Single-ended interfaces benefit from their simplicity and their low implementation cost, but have three main drawbacks:
1) They are highly sensitive to noise pick-up, because noise induced into the signal or ground paths adds directly to the receiver input, thus causing false receiver triggering.
2) Another concern is crosstalk, which is the capacitive and inductive coupling between adjacent signal and control lines, particularly at higher frequencies. Further, due to the physical differences between the signal trace and the ground plane, the transversal electromagnetic waves (TEM) generated in single-ended systems can radiate into the circuit environment, thus representing a significant source of electromagnetic interference (EMI) to adjacent circuits.
As is known in the art, differential signaling uses a signal pair consisting of two conductors—one for the forward, the other for the return current to flow. Each signal conductor possesses a common-mode voltage, VCM, superimposed with SO percent of the differential-driver output VOD, but of opposite polarity to one another. When the conductors of a differential pair are close to each other, electrically coupled external noise induced into both conductors equally appears as common-mode noise at the receiver input. Receivers with differential inputs are sensitive to signal differences only, but immune to common-mode signals. The receiver, therefore, rejects common-mode noise and signal integrity is maintained. Close electric coupling provides another benefit. The currents in the two conductors, being of equal amplitude but opposite polarity, create magnetic fields that cancel each other. The TEM waves of the two conductors, now being robbed of their magnetic fields, cannot radiate into the environment. Only the far smaller fringing fields outside the conductor loop can radiate, thus yielding significantly lower EMI.
As is also known in the art, it is sometimes necessary to connect differential signal circuit (such as a driver fabricated using, for example, a silicon-based technology) to single-ended circuit (a receiver such fabricated using III-V-based technology). Proper interface between the differential signal circuit and the single-ended circuit is critical to system performance, even more so in multi-chip modules with large distances between the chips.
One technique used to connect differential circuitry, such as a driver, to single ended circuitry, such as a receiver, is shown on FIG. 1. Here, a differential driver includes a pair of bipolar transistors formed on a silicon chip having collector electrodes providing a differential signal and emitter electrodes connected together and then to a reference potential, here ground, through a current source, as shown. One of the collector outputs is connected to one end of a single ended transmission line (TL) and the other collector is connected to the reference potential through a resistor, as shown. The other end of the single ended transmission line is connected to the input of a receiver. The receiver includes a Field Effect Transistor FET formed on a column III-V semiconductor chip. The FET has a gate electrode connected to the single ended transmission line through an input impedance matching network (IMN), as shown, an a source electrode connected to the reference potential, as shown. The disadvantages are: Cross-talk between adjacent TLs; Driver sensitivity to noise in bias lines (“ground bounce”); Additional TL loss due to radiation; and one-half of the signal is lost in the termination within the driver
Another interface uses a balun (passive balanced-to-unbalanced transformer) as shown in FIG. 2. Depending on where the balun is located (it could be on either of the two chips or as a separate component), the driver and receiver are connected through either a single-ended transmission line (TL) or a differential TL (Diff TL) or combination of the two. Disadvantages in using a balm are: There is some cross-talk between adjacent TL and radiation loss if single ended transmission line (S-E TL) is used; Balm loss and bandwidth limit; and Balun complexity and size (especially at low frequencies.
Yet, another interface uses an active converter, as shown in FIG. 3. Depending where the active converter is located (it could be on either of the two chips or as a separate component), the driver and receiver are connected through either a single-ended TL or a differential TL or combination of the two. Disadvantages: Some cross-talk between adjacent TL and radiation loss if single ended TL is used; The active converter's bandwidth limit and its DC power consumption of the DC voltage source.